1. Field of the Invention
The present invention relates to detection of a relative position of a wire electrode with respect to an electroconductive object, representing a boundary between a contact state and a noncontact state between the wire electrode and the electroconductive object (workpiece, jig or the like), in a wire cut discharge machine. The relative position of the wire electrode can be used for determining a reference position for machining.
2. Description of the Prior Art
In machining by the wire cut discharge machine, a machining reference position for obtaining the relative position of the wire electrode with respect to the workpiece (or a detection jig for positioning used instead of the workpiece) is set. For this purpose, the wire electrode is relatively moved with respect to the workpiece so as to bring the workpiece into contact with the workpiece, and the relative position (contact position) of the wire electrode with respect to the workpiece at the time of contact is determined, and the machining reference position is generally determined based on the contact position.
For the machining reference position, the contact position between the wire electrode and the workpiece may be directly used, or another position having particular relation with the contact position may be used. For example, the wire electrode is made to pass through a round-hole formed corresponding to the machining start portion of the workpiece, three points are determined as a contact positions between the inner wall of the round-hole and the wire electrode by the relative movement in three directions, and the central position of a circular arc passing through these three points may be adopted as a machining reference position.
In order to accurately determine the machining reference position, the contact position between the wire electrode and the workpiece must be determined accurately. Generally, the detection of the contact position is performed by applying a detection voltage to between the wire electrode and the workpiece, and detecting a difference or transition of the detection voltage corresponding to the contact or noncontact state. In actually determining the contact position as described below, however, since a gray zone indicating an intermediate state (unstable state) between contact and noncontact states occurs, it is difficult to determine the contact position with good reproducibility.
In order to determine the contact position between the wire electrode and the workpiece, at first, the wire electrode is placed at a position reliably away from the workpiece (in the noncontact state), and the wire electrode is relatively moved toward the workpiece, with a contact detection voltage applied to between these from the position. By detecting that the wire electrode has been shifted from the noncontact state to the contact state with respect to the workpiece, from the transition of the voltage value for detecting the contact, the relative position of the wire electrode with respect to the workpiece at the time of detection is determined as a “contact position”. The shift of the wire electrode from the noncontact state to the contact state is judged herein by detecting the fact that the detection voltage applied to between the wire electrode and the workpiece changes from a value corresponding to the noncontact state to a value corresponding to the contact state.
However, the change of the detection voltage at the time when the wire electrode shifts from the noncontact state to the contact state is unstable and the reproducibility is poor. It is considered that this is mainly attributable to the fact that when the wire electrode shifts from the noncontact state to the contact state, the wire electrode undergoes an unstable state accompanied with vibration. Since the wire electrode is continuously supplied at a considerably high feed rate so that the contact portion between the wire electrode and the workpiece is renewed at all times, in order to reduce the influence of mechanical and electrical damages on the wire electrode, which occur at the time of detecting the contact, vibration occurs inevitably in the wire electrode. Such vibration of the wire electrode make unstable the contact state between the wire electrode and the workpiece.
Further, the detection voltage may not clearly show a change, even if the wire electrode and the workpiece mechanically contact with each other, because an insulating film due to fats and oils or electrolysis exists on the surface of the wire electrode and the workpiece, and the contact pressure of the wire electrode against the workpiece is small even when the wire electrode completely comes in contact with the workpiece.
The observation of the transition of the detection voltage in the process in which the wire electrode relatively moves toward the workpiece from a position away from the workpiece schematically shows the followings. When the wire electrode relatively moves toward the workpiece from a state completely away from the workpiece, and the contact with the workpiece begins unstably, an initial drop in the detection voltage is observed. It is considered that this initial drop in the detection voltage corresponds to the start of contact of the wire electrode with respect to the workpiece. For a while thereafter, the detection voltage fluctuates frequently. Then, no change is observed with the detection voltage remaining low. It is considered that this state in which there is no change with the detection voltage being low expresses the complete contact state.
As described above, the existence of the unstable condition between contact and noncontact states, between the state in which the wire electrode is completely away from the workpiece, and the state in which the wire electrode completely comes in contact with the workpiece, makes it difficult to clearly specify the above contact position, that is, the relative position of the wire electrode at a point in time when the wire electrode shifts from the noncontact state to the contact state with respect to the workpiece. Therefore, when a process for making the wire electrode relatively approach and come in contact with the workpiece is executed for a plurality of times, while applying and monitoring the detection voltage, various transition of the detection voltages, which differ for each time, are observed, and hence the contact position cannot be obtained with high reliability. In other words, it is difficult with the conventional method to obtain with good reproducibility the relative position of the wire electrode with respect to the workpiece, corresponding to the boundary between the contact state and the noncontact state of the wire electrode and the workpiece.
On the other hand, for example, there is a method of using the statistical technique in which the contact position detection operation is repeated to average the obtained contact position detection values. However, this method does not essentially solve the above problem, and if it is tried to increase the precision, while ensuring the sufficient reproducibility at the detection position, it is necessary to increase the number of repetitions of the detection operation, thereby decreasing the efficiency.